Polypropylene with high melt strength and drawability

ABSTRACT

Use of a multimodal polypropylene blend in melt processing wherein for enhancing a compromise between melt strength and drawability the blend has a dispersion index of at least 8 and a ratio Mz/Mn of at least 10.

The present invention relates to polypropylene.

Isotactic and syndiotactic polypropylene, and blends thereof, are knownfor use in a number of different applications. For example,polypropylene is used for the manufacture of spun fibres, blown films,extruded profiles and foams. In such applications in which thepolypropylene is processed while molten, it is desirable for the polymerto have a high melt strength. For some applications, for example fibrespinning and film blowing, as well as having a high melt strength thepolypropylene is required to have a high drawability. A high drawabilitynot only enables the fibres or films to be produced at high speedwithout fracture, but also enables finer diameter fibres and thinnerfilms to be manufactured.

There tends to be a compromise between high melt strength anddrawability. Thus some known polypropylenes have high melt strength butlow drawability. This makes them unsuitable for drawing fibres,particularly of small diameter.

A polymer melt having high melt strength at high shear rates refers to amelt that becomes stiffer and stronger when stretched, rather than onethat thins out and breaks when stretched. This stiffening upon drawingis commonly called strain-hardening. Polypropylene processing operationswhere melt strength plays an important role include blow moulding,extrusion coating, thermoforming, fibre spinning and foam extrusion. Inthermoforming, a poor melt strength results in a sagging phenomenon. Infibre spinning, a poor melt strength can result in undesired movementsof the fibres due to transverse forces, for example by cooling air,which ultimately can lead to “married” fibres and fibre breakage. On theother hand, a too-high melt strength will limit the achievement of lowtitre fibres. Accordingly, a correct balance between melt strength anddrawability is desirable. For blown (biaxially oriented) or cast filmsalso, a correct balance between melt strength and stretchability is veryimportant. In foam extrusion, a poor melt strength results in cellrupture and non-uniform cell structure. For such an application, a poordrawability will limit the fineness of the walls.

Several solutions have been proposed in the prior art to increase themelt strength of polypropylene. For example, polymers with long chainbranching tend to exhibit good melt strength. For isotacticpolypropylene, this can be achieved by irradiation or by reactiveextrusion processes, such as disclosed in U.S. Pat. Nos. 5,047,446,5,047,485 and 5,541,236. The limitation of these processes is thesignificant reduction of drawability occurring at the same time as meltstrength increases. In addition, the irradiation process is expensive.It has also been proposed to blend isotactic polypropylene withadditives, such as high molecular weight acrylates, to increase the meltstrength, as disclosed for example in EP-A-0739938. The same results canbe achieved by blending with isotactic polypropylene polyethylene havinghigh melt strength or fillers. These processes are limited by the strongmodification by the additives of the intrinsic properties of theisotactic polypropylene.

It is also known from the literature that the melt strength of isotacticpolypropylene is solely determined by its weight average molecularweight (Mw) (A. Gijsels Ind. Polym. Process., 9, 252 (1994)).

U.S. Pat. No. 5,549,867 relates to a melt spinning process forpolyolefin resins in which a blended resin includes a relatively smallportion of a low molecular weight high melt flow rate narrow molecularweight distribution polyolefin resin with a larger portion of a misciblehigh molecular weight, low melt flow rate and typically narrow molecularweight distribution polyolefin resin. It is disclosed that the enhancedmolecular weight distribution polyolefin blended resin has a variety ofproperty parameters, including a molecular weight distribution breadthMz/Mn of between 7.2 and 10, a flow rate ratio of less than 15.5 and apower law index at 20 seconds⁻¹ of between 0.70 and 0.78 and either aZ-average molecular weight Mz of between 400,000 and 580,000, or asecond order constant b₂ determined from the regression analysisviscosity equation of between −0.029 and −0.047 or both, and unless bothof the Mz and b₂ parameters is within said ranges, a die swell B² ofbetween 1.6 and 2.0 and a spinnability factor ln (B²)/MFR of betweenabout 0.08 and about 0.026.

U.S. Pat. No. 5,494,965 discloses a process for manufacturing bimodalolefin polymers and copolymers. However, the specification does notaddress the problems of drawing polypropylenes.

U.S. Pat. No. 5,578,682 discloses the bimodalisation of a polymermolecular weight distribution by using grafting and scission agents.

EP-A-0310734 discloses catalyst systems for producing polyolefin havinga broad molecular weight distribution, in particular a multimodalmolecular weight distribution. This specification does not address theproblems of drawability of polypropylenes.

It is an aim of the present invention to provide polypropylene, whichmay be isotactic, syndiotactic or a blend of isotactic and syndiotacticfractions, which provides improved properties such as melt strength anddrawability. It is also an aim of the present invention to provide suchpolypropylene which can be used in processing applications which requirethe polypropylene to be processed from the melt, for example at highshear rates, typically in fibre spinning. It is a further aim of thepresent invention to provide polypropylene which has an improvedcompromise between melt strength and drawability.

Accordingly the present invention provides the use of a multimodalisotactic polypropylene blend in melt processing wherein for enhancing acompromise between melt strength and drawability the blend has adispersion index of at least 8 and a ratio Mz/Mn of at least 10.

The present invention also provides a method of enhancing a compromisebetween melt strength and drawability in melt processing of apolypropylene, the process including providing a multimodalpolypropylene blend having a dispersion index of at least 8 and a ratioMz/Mn of at least 10.

The present invention further provides a method of melt processing apolypropylene blend, the method comprising providing a multimodalpolypropylene blend, selecting the blend to have a dispersion index offrom 8 to 70 and a ratio Mz/Mn of at least 10 thereby enhancing acompromise between melt strength and drawability, and processing theblend in the melt by drawing the blend to form a solid product.

In this specification, the dispersion index (D) (also known as thepolydispersity index) is the ratio between the weight average molecularweight (Mw) and the number average molecular weight (Mn). The ratioMz/Mn is the molecular weight distribution breadth. Mz is the z-averagemolecular weight, defined as ΣNiMi³/ΣNiMi² over all i.

The multimodal blend is preferably bimodal, but may alternatively betrimodal, tetramodal, etc. The blend of the fractions may be obtained byphysical blending or chemical blending, for example chemical blendingusing two reactors in series or chemical blending using one reactor withspecific dual-type catalysts. The polypropylene fractions may becomposed of homopolymer or copolymer and may be made using differingcatalysts, for example Ziegler-Natta catalysts or metallocene catalysts.

Preferably, the dispersion index is greater than 15. The dispersionindex may be up to about 70.

The molecular weight distribution breadth is not especially limited,provided that it is 10 or above. Preferably, the molecular weightdistribution breadth (Mz/Mn) is from 50-150.

The blend may comprise from 20 to 80 wt % of a first high molecularweight fraction and from 80 to 20 wt % of a second low molecular weightfraction.

Preferably, the blend comprises from 50 to 70 wt % of the first fractionand from 50 to 30 wt % of the second fraction. More preferably, theblend comprises from 55 to 65 wt % of the first fraction and from 45 to35 wt % of the second fraction.

Preferably, the ratio of the melt flow indexes of the first and secondfractions is at least 5. Typically, the first fraction has a melt flowindex of less than 5 dg/min and the second fraction has a melt flowindex of from 60 to 1000 dg/min.

Optionally, the blend has been formed by reactive extrusion of a mixtureof the first and second fractions together with a mixture of a chainscission agent and a chain grafting agent. The chain scission agent maycomprise 2,5-dimethyl-2,5-di(tert-butylperoxy) hexane. The chaingrafting agent may be selected from allyl methacrylate and divinylbenzene.

The first and second fractions, and the blend, are preferably comprisedof polypropylene homopolymer. Alternatively, the first and/or secondfractions may se comprised of polypropylene copolymer.

The present invention also relates to the use of the polypropylene forforming fibres, foams, films, thermoformed articles and extrudedproducts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a gel-permutation chromatogram of a bimodal isotacticpolypropylene having a melt flow index of 11.5 dg/min;

FIG. 2 is a gel-permutation chromatogram of a bimodal polypropylenehaving a melt flow index of 6.9 dg/min; and

FIG. 3 is a gel-permutation chromatogram of a bimodal polypropylenehaving a melt flow index of 1.1 dg/min.

The present invention is predicated on the discovery by the inventorsthat the mechanical properties, in particular the melt strength anddrawability of polypropylene can be improved by increasing the molecularweight distribution of the multimodal polypropylene by providing a highdispersion index (D), which is the ratio Mw/Mn (where Mw is the weightaverage molecular weight and Mn is the number average molecular weight).The melt strength is typically measured by measuring the force requiredin order to pull a fibre from an extruded melt onto a rotating wheelunder given conditions. A polypropylene having higher melt strength isgenerally more reliably processed from the melt, for example in thespinning of fibres, in the blowing of films, in thermoforming, and inthe extrusion of profiles, such as tubes or pipes. Generally, as themelt strength increases, the tendency for the molten material to breakor deform decreases.

The melt strength tends to increase with a decrease in the melt flowindex (MFI) of the polypropylene. In this specification, the MFI valuesare determined using the procedures of ASTM D1238 using a load of 2.16kg at a temperature of 230° C.

As well as having high melt strength, it is desired for fibre spinning(and film blowing) that the polypropylene has a high drawability. A highdrawability represents the ability of the material to be stretched intoa small diameter fibre (or a thin film) at high speed, i.e. at highstrain rates. Typically, the drawability of the polypropylene isdetermined by wrapping a fibre around a wheel rotating at constantacceleration during spinning of the fibre and measuring the maximumangular speed, in units of revolutions per minute, up to rupture of thefilament. With increasing drawability, the speed of drawing can increaseprior to rupture, thereby enabling even finer filaments to bemanufactured.

Thus the present inventors have found that by providing a polypropyleneblend with a dispersion index of at least 8, the melt strength anddrawability together can be sufficiently high to yield a good compromisebetween them, enabling the blend to have particularly advantageousapplication for fibre spinning.

The present invention will now be described by way of example only, withreference to the accompanying drawings, in which: FIGS. 1 to 3 are gelpermeation chromatograms (GPCs) of isotactic polypropylene resins inaccordance with three different embodiments of the invention.

Referring to FIG. 1, there is shown a GPC chromatogram of a bimodalisotactic polypropylene in accordance with a first embodiment of thepresent invention. The bimodal isotactic polypropylene has a melt flowindex of 11.5 dg/min. The molecular weight distribution (A) of thebimodal isotactic polypropylene is such that Mw is 279 kDa, Mn is 30 kDaand the dispersion index is accordingly 9.3. The bimodal isotacticpolypropylene is formed as a physical blend of two isotacticpolypropylene homopolymer fractions. The first fraction is a highmolecular weight fraction (B) having an MFI of 2.3 dg/min and comprising55 wt % of the bimodal isotactic polypropylene. The second fraction is alow molecular weight (C) fraction having an MFI of 72 dg/min andcomprising 45 wt % of the bimodal isotactic polypropylene. The highmolecular weight fraction has an Mw of 419 kDa, an Mn of 49 kDa and adispersion index of 8.6, and the low molecular weight fraction has an Mwof 146 kDa, an Mn of 21 kDa and a dispersion index of 7.0.

FIG. 2 is a GPC chromatogram of a bimodal isotactic polypropylene inaccordance with a second embodiment of the present invention. Thebimodal isotactic polypropylene has a melt flow index of 6.9 dg/min. Themolecular weight distribution (A) of the bimodal isotactic polypropyleneis such that Mw is 363 kDa, Mn is 26 kDa and the dispersion index isaccordingly 14.1. The bimodal isotactic polypropylene is formed as aphysical blend of two isotactic polypropylene homopolymer fractions, thefirst being a high molecular weight fraction (B) having an MFI of 0.8dg/min and comprising 57 wt % of the bimodal isotactic polypropylene,and the second being a low molecular weight fraction having an MFI of350 dg/min and comprising 43 wt % of the bimodal isotacticpolypropylene. The high molecular weight fraction has an Mw of 568 kDa,an Mn of 73 kDa and a dispersion index of 7.8, and the low molecularweight fraction (C) has an Mw of 99 kDa, an Mn of 16 kDa and adispersion index of 6.2.

FIG. 3 is a GPC chromatogram of a bimodal isotactic polypropylene inaccordance with a third embodiment of the present invention. The bimodalisotactic polypropylene has a melt flow index of 1.1 dg/min. Themolecular weight distribution (A) of the bimodal isotactic polypropyleneis such that Mw is 671 kDa, Mn is 27 kDa and the dispersion index isaccordingly 24.9. The bimodal isotactic polypropylene is formed as aphysical blend of two isotactic polypropylene homopolymer fractions, thefirst being a high molecular weight fraction (B) having an MFI of 0.06dg/min and comprising 55 wt % of the bimodal isotactic polypropylene,and the second being a low molecular weight fraction (C) having an MFIof 450 dg/min and comprising 45 wt % of the bimodal isotacticpolypropylene. The high molecular weight fraction has an Mw of 1460 kDa,an Mn of 142 kDa and a dispersion index of 10.2, and the low molecularweight fraction has an Mw of 95 kDa, an Mn of 15 kDa and a dispersionindex of 6.3.

It may be seen for FIGS. 1 to 3 that in accordance with the embodimentsof the invention, each bimodal isotactic polypropylene blend is composedof two initial isotactic polypropylene fractions. The fractions areselected so as to provide a minimum dispersion index (D) of 8 in thebimodal isotactic polypropylene blend. The dispersion index D may be upto about 70 for blends in accordance with the invention. The fractionsare also selected so as to have specific respective melt flow indexes,thereby to provide a melt flow index differential between the twofractions, to provide the required minimum dispersion index in theultimate blend. In this way, the molecular weight distribution of theblend is broadened, which has been found by the inventors to provideincreased melt strength at any given melt flow index for the blend. Inaddition, as the melt flow index of the blend increases, this also tendsto decrease the melt strength.

In the blending operation using reactive extrusion, a mixture of a chainscission agent and a chain grafting agent may be employed.

The chain scission agent may for example comprise a peroxide compound,typically 2,5-dimethyl-2,5-di(tert-butylperoxy) hexane. The use of sucha chain scission agent combined with a grafting agent tends to increasethe degree of branching of the molecules in the high molecular weightfraction, thereby increasing the molecular weight distribution of theultimate polypropylene, thereby in turn yet further increasing the meltstrength.

The grafting agent may be a bi- or multifunctional grafting agent,typically allyl methacrylate or divinyl benzene. The grafting agentpromotes cross-linking of the branches formed by the chain scissionagent. This increases the melt strength but tends to reduce thedrawability or spinnability of the polypropylene.

Typically, the extrusion temperature is around 220° C. When a chainscission agent such as 2,5-dimethyl -2,5-di(tert-butylperoxy) hexane isemployed, this is employed in an amount of around 55 ppm based on t heweight of the blend. When a grafting agent is employed, such as allylmethacrylate, this is typically employed in an amount of around 750 ppmbased on the weight of the blend.

The flexural modulus of the polypropylenes is a complex functiondepending upon several parameters, not only the dispersion index D butalso for example the melt flow index, the xylene solubles and thecrystallinity of the polymer. The present inventors have also found thatthe flexural modulus E of the polypropylenes in accordance with theinvention tends to increase with increasing dispersion index at constantmelt flow index and xylene solubles.

Preferably, the isotactic polypropylenes of the present invention areproduced with Ziegler-Natta catalysts using a phthalate as an externalelectron donor, see for example EP-A-0045976, EP-A-0045977 and U.S. Pat.No. 4,544,171. The phthalate may be replaced by a 1,3 diether compound,see for example U.S. Pat. Nos. 4,971,937, 4,978,648 and 5,095,153. Theisotactic polypropylene can also be produced with metallocene catalysts,see for example EP-A-0416566, EP-A-0399348 and EP-A-0336128. Theinvention also applies to syndiotactic polypropylene, or to a blend ofisotactic and syndiotactic polypropylene obtained by physical mixing orchemical mixing using for example a metallocene catalyst, as disclosedfor example in U.S. Pat. No. 5,036,034. In addition, the polypropylenemay be treated with a nucleating agent, typically lithium benzoate, fornucleation of crystallites in the polypropylene.

The polypropylene may be a homopolymer, a random copolymer containingethylene and a higher alpha-olefin, or a heterophasic block copolymer ofethylene and a higher alpha-olefin.

The present invention will now be described further with reference tothe following non-limiting Examples.

EXAMPLES 1 to 4

For each of Examples 1 to 4, a bimodal isotactic polypropylene blend wasproduced by blending together a high molecular weight component and alow molecular weight component in a screw extrusion apparatus operatedunder nitrogen gas at a temperature of around 220□C. Table 1 specifiesthe composition and properties for both the high molecular weight andlow molecular weight components, and the ultimate blend for each ofExamples 1 to 4. The molecular weight distributions of the blends andcomponents of Examples 2 and 4 are shown respectively in FIGS. 2 and 3.

For each of Examples 1 to 4, the melt strength was tested by measuringthe force for a fibre which is pulled from a melt. In this specificationthe melt strength was determined in a laboratory using a CEAST rheometer(Rheoscope 1000) equipped with a capillary die and a rotating wheel as atake up device. With this set up, molten polymer is extruded byapplication of a pressure resulting from the displacement of a piston.The molten extrudate is uniaxially stretched before crystallisation bywrapping the fibre around the rotating wheel. In this test, the pistondisplacement rate is fixed, and the speed of the rotation take-up wheelis linearly changed, i.e. with constant acceleration, until the fibre,becoming very thin, breaks. The tensile force is recorded during thetest. The melt strength is defined as the maximum tensile forcecorresponding to the breaking of the fibre. The tests were run understandard conditions as follows: the cylindrical die had alength/diameter ratio of 5 mm to 1 mm; the diameter of the rotatingwheel was 12 cm; the extrusion temperature was 250° C.; the displacementratio of the piston was 2 mm/min, the extrudate throughput was 2.36mm³/min and the acceleration of the rotating wheel was 10 rpm/100s or0.000628 m/s². The drawability is defined as the titre at break underthe same conditions. The correspondence between the angular speed of thewheel (V) expressed in rpm and the titre (expressed in denier) is thefollowing: titre at break=3384.4 ρ/V where ρ is the polymer density at250° C. The results are also specified in Table 1.

It may be seen for each of Examples 1 to 4 that the dispersion index D,which is the ratio Mw/Mn, is greater than 8 and the melt flow indexvaries from 1.1 to 6.9 dg/min. The melt strength varied from 2.8 mN to15.5 mN. In combination with the melt strength values, the drawabilityof the polymers of each of Examples 1 to 4 is high, the filamentbreaking at least 260 rpm at a temperature of 250° C.

COMPARATIVE EXAMPLE 1

As a comparison, the corresponding properties of a commerciallyavailable isotactic polypropylene resin with a high degree of long chainbranching were tested. The resin is sold under the trade name ProfaxPF814 by the company Montell North America Inc. of Wilmington, Del.,USA. It may seen that while the resin of Comparative Example 1 had avery high melt strength, being around 3 times that of the maximum meltstrength of Examples 1 to 4, particularly for that of Example 4,nevertheless the drawability of the commercial resin was very low, withthe filament breaking at a speed of only 9 rpm at 250° C.

COMPARATIVE EXAMPLE 2

As a further comparison, the corresponding properties of a monomodalpolypropylene resin were tested and the results are shown in Table 1.

Table 1 shows that the multimodal polypropylene resins in accordancewith the invention have a good compromise between high melt strength andhigh drawability. This is achieved by for example a blend of high andlow molecular weight components having a high dispersion index of atleast 8. This makes the resins of the invention particularly suitablefor fibre spinning and film blowing. The resins of the invention alsohave utility in lower shear rate processes, such as the formation offilms and in extrusion forming processes. The multimodal polypropylenein accordance with the invention can be stretched more than the highmelt strength polypropylene with long chain branching of ComparativeExample 1. The propylene of the Examples had a higher drawability thanthe polypropylene of Comparative Example 2. Thus the present inventionprovides polypropylene blends having an improved compromise of high meltstrength and extensibility in the melt.

TABLE 1 High Mw Low Mw component component Blend MFI MFI MFI Melt Draw-wt (dg/ wt (dg/ (dg/ D strength ability Mn Mw Mz % min) D % min) D min)(Mw/Mn) (mN) (rpm) (kDa) (kDa) (kDa) Mz/Mn Example 1 65 0.8 7.8 35  636.2 2.9 10.1 4.4 >300 44.7 453 2313 51.7 Example 2 57 0.8 7.8 43 350 6.26.9 14.1 3.0 >300 25.7 363 2255 87.7 Example 3 65 1.6 7.1 35 450 6.3 6.910.8 2.8 >300 31 336 1651 53.3 Example 4 55  0.06 10.2  45 450 6.3 1.124.9 15.5 260 27 671 4057 150.3 Comparative — — — — — — 3.0 9.2 52 944.1 404 1680 38.1 Example 1 Comparative — — — — — — 1.0 7.2 8 240Example 2

What is claimed is:
 1. A method of melt processing a polypropyleneblend, the method comprising providing a bimodal polypropylene blend ina molten state, said blend comprising from 50 to 70 wt % of a first highmolecular weight fraction and from 50 to 30 wt. % of a second lowmolecular weight fraction and having a melt dispersion index of from 8to 70 and a ratio Mz/Mn of at least 10 thereby enhancing a compromisedbetween melt strength and drawability, and processing the blend in themelt by drawing and cooling the blend to form a solid product.
 2. Amethod according to claim 1 wherein the dispersion index is greater than15.
 3. A method according to claim 1 wherein the ratio of Mz/Mn is from50-150.
 4. A method according to claim 3 wherein the dispersion index isgreater than
 15. 5. A method according to claim 1 wherein the blendcomprises from 55 to 60 wt. % of the first fraction and from 45 to 35wt. % of the second fraction.
 6. A method of melt processing apolypropylene blend, the method comprising providing a multimodalpolypropylene blend in a molten state, said blend having a meltdispersion index of from 8 to 70 and a ratio Mz/Mn of at least 10thereby enhancing a compromise between melt strength and drawability,and processing the blend in the melt by drawing and cooling the blend toform a solid product wherein the blend has been formed by reactiveextrusion of a mixture of at least two fractions together with a mixtureof a chain scission agent and a chain grafting agent.
 7. A methodaccording to claim 6 wherein the chain scission agent comprises2,5-dimethyl-2,5-di(tert-butylperoxy) hexane.
 8. A method according toclaim 6 wherein the chain grafting agent is selected from the groupconsisting of allyl methacrylate and divinyl benzene.
 9. A polypropyleneblend useful in melt processing and providing for enhancing a compromisebetween melt strength and drawability, said blend having a dispersionindex of at least 8 and a ratio Mz/Mn of at least 10, wherein the blendis bimodal and comprises from 50 to 70 wt. % of a first high molecularweight fraction and from 50 to 30 wt. % of a second low molecular weightfraction.
 10. A multimodal polypropylene blend according to claim 9wherein the ratio of the melt flow indexes of the first and secondfractions is at least
 5. 11. A multimodal polypropylene blend accordingto claim 9 wherein the blend comprises from 55 to 65 wt. % of the firstfraction and from 45 to 35 wt. % of the second fraction.
 12. Amultimodal polypropylene blend according to claim 11 wherein the ratioof the melt flow indexes of the first and second fractions is at least5.
 13. A multimodal polypropylene blend useful in melt processing andproviding for enhancing a compromise between melt strength anddrawability, said blend having a dispersion index greater than 15 and aratio Mz/Mn of a least
 10. 14. A multimodal polypropylene blend usefulin melt processing and providing for enhancing a compromise between meltstrength and drawability, said blend having a dispersion index of atleast 8 and a ratio Mz/Mn of from 50-150.
 15. A multimodal polypropyleneblend according to claim 14 wherein the dispersion index is greater than15.